Acoustic particle concentration measuring instrument and method

ABSTRACT

A liquid or solid medium is acoustically examined to detect or measure the concentration of a selected entrained particle matter of different acoustic impedance than the medium and given average particle size by transmitting through the medium to acoustic transducer means acoustic wave energy of a selected frequency such that the wave energy is attenuated by scattering and diffraction by all the constituents of the medium down to and including the particle size of the selected matter, and comparing the transducer output voltage produced by the incident wave energy with a reference transducer output voltage produced by acoustic wave energy which is attenuated by all the constituents except the selected particulate matter to obtain a resultant voltage representing the concentration of the selected matter. This resultant voltage may be utilized to drive a meter or recorder for displaying concentration and/or an alarm for signalling a critical concentration. One disclosed embodiment of the invention is a single frequency system for use in applications where the medium contains only the selected particulate matter to be detected and employs an external reference voltage source. A second disclosed embodiment is a multiple frequency system for use where the medium contains more than one particulate matter and utilizes a transducer output as a reference voltage. A primary application of the invention involves measuring the concentration of oil in water.

United States Patent [191 Johnson et al.

1 1 Jan. 16,1973

[54] ACOUSTIC PARTICLE CONCENTRATION MEASURING INSTRUMENT AND METHOD 9 [75] Inventors: Robert L. Johnson, Marina Del Rey;

Kenneth R. Overoye; Pravin G. Bhuta, both of Torrence, all of Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif.

[22] Filed: March 25, 1971 [21] Appl. No.: 127,872

[52] US. Cl ..73/6l R, 73/677, 340/236 [51] Int. Cl. ..'....G01n 29/02 [58] Field of Search ..73/61 R, 61.1 R, 67.6, 67.8; i 340/236, 261

[56] References Cited I UNITED STATES PATENTS 4/1942 Firestone ..73/67.8 12/1960 Heller 3,093,998 6/1963 Albertson et a1. ..73/61 R 3,269,172 8/1966 McGaughey.... ..73/6l R 3,359,788 12/1967 Colvin ....7-3/6l.l R 3,553,636 l/l971 Baird ..73/6l R X FOREIGN PATENTS OR APPLICATIONS I MC DRIVER 5/1966 u;s s.R. ..73/61R 180,844 5/1966 U.S.S.R.;..' ..73/61R' Primary ExaminerLouis J. Capozi Assistant Examiner -Joseph W. Roskos Attorney-Daniel T. Anderson et al.

[ ABSTRACT A-liquid or solid medium is acoustically examined to detect or measure the concentration of a selected entrained particle matter of different acoustic impedance than the medium and given average particle size by transmitting through the medium to acoustic transducer means acoustic wave energy of a selected frequency such that the wave energy is attenuated by scattering and diffraction by all the constituents of the medium down to and including the particle size of the selected matter, and comparing the transducer output voltage produced by the incident wave energy with a reference transducer output voltage produced by acoustic wave energy which is attenuated by all the constituents except the selected particulate matter to obtain a resultant voltage representing the concentration of the selected matter. This resultant voltage may be utilized to drive a meter or recorder for displaying concentration and/or an alarm for signalling a critical concentration. One disclosed embodiment of the invention is a single frequency system for use in applications where the medium contains only the selected particulate matter to be detected and employs an external reference voltage source. A second disclosed embodiment is a multiple frequency system for use where the medium contains more than one particulate matter and utilizes a transducer output as a reference voltage. A primary application of the invention involves measuring the concentration of oil in water.

13 Claims, 2 Drawing Figures DETECTOR CHART RECORDER SUBTRACTOR VOLT-METER ALARM 0c. OFF- SET VOLTAGE 26 2o 3s v .PATENTEDJAN 15 ma 3.710.615

34 J CH RT RECORDER DETECTOR SUBTRACTOR VOLT-METER ALARM D.C. OFF- SET VOLTAGE 2s 20 38 DETECTOR DETECTOR 51 SUBTRACTOR IQGIEIIDEDZIJ 4 I08 7 2 I |voLTMETER| CHART RECORDER Robert L. Johnson Kenneth R. Overoye Provln G. Bhufo INVENTORS ATTORNEY ACOUSTIC PARTICLE CONCENTRATION MEASURING INSTRUMENT AND METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the field of acoustic measurement and more particularly to a method of and an instrument for acoustically examining aliquid. or solid medium to detect or measure the concentration of entrained particulate matter of dif- 1 the present disclosure, the expression particulate matter refers to either a solid or liquid in particle or droplet form which-may or may not normally be present in the medium and which may beeither in the nature of a desirable or essential constituent or an undesirable or foreign constituent or contaminant. Acoustic. particle concentration measuring methods and instruments are known. A selection of prior art us. Pat. Nos; in this general field is set forth below:

As will appear from the ensuing description, the present acoustic measuring method and instrument is capable of many diverse applications. A primary use of the invention, however, involves measuring the concentration of oil in water. The invention will be disclosed in connection with this particular application.

One of the major sources of pollution of navigable waters by oil is discharge of oily waterby ships. The

main source of the oily water is ballast and. bilge discharge and tanker washings. As a result of the i963 lnternational Conference on Oil Pollution, a limit of 100 ppm of oiland water discharged from ships was established. Given that the intent of both government and industry is to comply with this agreement its-practi-. cal implementation is very difficult due to the lack of a reliable technique for measuring the oil content in discharged water. Besides ballast and bilge discharge from ships, oil refineries must discharge large quantities of water which might containoil, In light of the present public attitude toward pollution, good business practice dictates that refining companies restrict their discharge to water containing less than l'OO ppm oil.

Numerous techniqueshave been tried in an attempt ing of electromagneticradiation in both the. infrared and ultraviolet bands. 7

None of the systems based on these techniques .per-

.form satisfactorily at present and most proposed systems suffer disadvantages inherent in the basic ment ofan improved system.

SUMMARY or THE INVENTION The present invention provides a novel acoustic wave energy to a corresponding fluctuating electrical output voltage. Also included in the instrument are means for producing a reference voltage and means for.

comparing the voltages to obtain a resultant voltage representing the difference between the output and reference voltages.

The acoustic source is driven at a selected acoustic frequency such that acoustic wave energy is transmitted through the acoustic medium to the receiver witha half wavelength approximating the mean diameter of the selected particles to be detected or whose concentration is to be measured. Accordingly, during its passage through the medium, the acoustic wave age comparison means of the instrument, thus' represent the attenuation produced by and therefore the concentration of the selected particles in the medium. This resultant voltage may be utilized to drive a meter or recorder for displaying the particle concentrationand/orto actuate an alarm for signaling a critical concentration. 7'

Two inventive embodiments are disclosed-One embodiment is a single frequency system for use inapplications where the acoustic medium may contain only the selected particles to be detected or measured such as oil droplets in water. in this system only a single acoustic frequency, having a half wavelength approximating the mean diameter of the particles, is transmitted'through the medium. The output of the acoustic receiver is amplified and converted to a corresponding dJc. output voltage.- The referencevoltage is a dc voltageprovided by an external reference voltage source which is adjusted'to a reference voltage level equal to the receiver d.c. output voltage when the acoustic medium is totally free of the selectedparticles to be detected. Accordingly, the difference between the eternal reference voltage and the instantaneous receiver d.c.

output voltage representsthe attenuation produced by and hence the concentration of the selected particles. The voltage comparison means is an electrical differencing circuit which'produces a difference voltage equal to the difference between the references and receiver output voltages. This difference voltage drives a meter and recorder for displaying concentration and an alarm for signaling a critical concentration.

The second disclosed system is a multiple frequency system for use in applications where the acoustic medium may contain particles of different mean diameter, such as oil droplets and larger particles of sand, rock, dirt or the like. In this instrument, at least two different acoustic frequencies are transmitted through the acoustic medium. The half wavelength of the higher frequency approximates the mean diameter of the selected particles to be detected or measured. The half wavelength of the lower frequency is substantially greater than the mean diameter of the selected particles but equal to or less than the next larger particle mean diameter. The acoustic wave energy of the higher frequency is thus attenuated by all the entrained particles down to the diameter of and including the selected particles. The acoustic wave energy of the lower frequency is attenuated by all the particles of larger diameter than the selected particles. Accordingly, the difference in amplitude of the acoustic wave energy of the two frequencies arriving at the acoustic receiver represents the attenuation produced by and hence the concentration of the selected particles. The receiver d.c. output voltage produced by the lower frequency is employed as a reference voltage which is compared to the receiver d.c. output voltage produced by the higher frequency to obtain a difference voltage representing the concentration of the selected particles for operating a meter, recorder and/or alarm.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a diagram of asingle frequency acoustic concentration measuring instrument according to the invention; and

FIG. 2 is a diagram of a dual frequency acoustic concentration measuring instrument according to the invention.

PREFERRED EMBODIMENTS OF THE INVENTION The acoustic instrument of FIG. 1 has an acoustic source 12 and an acoustic receiver 14 to be acoustically coupled to an acoustic wave transmission medium 16 to be acoustically examined for the purpose of detecting the presence or measuring the concentration in the medium of selected particles of different acoustic impedance than the medium and given mean particle diameter. The acoustic source and receiver are spaced so that acoustic wave energy from the source is transmitted through the acoustic medium to the receiver. The acoustic receiver produces a fluctuating output voltage proportional to the intensity or magnitude of the wave energy incident on the receiver. This output voltage is fed to a voltage comparison circuit 18 which receives a reference voltage from a reference voltage source 20. The comparison circuit produces a resultant voltage which represents the difference between the receiver output voltage and the reference voltage. As will appear presently, this resultant or difference voltage also represents the concentration of selected particles within the acoustic medium 16. Connected to and actuated by the output of the comparison circuit 18 are a meter 22 and recorder 24 for displaying the particle concentration and an alarm 26 for signaling a critical concentration.

Briefly, in operation of the acoustic instrument 10, the acoustic source 12 is operated to transmit through the acoustic medium 16 acoustic wave energy having a half wavelength approximating the mean diameter of the selected particles, such that the wave energy is attenuated by scattering and diffraction, by all the constituents of the medium down to the diameter of and including the selected particles to be measured. The reference voltage source 20 is adjusted to provide a reference voltage representing the output of the acoustic receiver with a known concentration of the selected particles in the medium, preferably zero concentration (i.e. pure medium). The comparison circuit 18 compares this reference voltage and the receiver output voltage when the acoustic wave energy is attenuated by the selected particles and produces a resultant or difference voltage representing the concentration of the selected particles in the medium.

The particular instrument 10 under discussion is a single frequency system which is intended for acoustically examining an acoustic medium which consists only of the medium itself and the selected particles, if any, to be detected or measured. In this case, the acoustic source '12 is initially operated at the selected acoustic frequency with a pure acoustic medium 16, i.e. a medium totally free of the selected particles, such that the receiver 14 produces an output voltage representing the acoustic wave energy incident on the receiver when no attenuation of the wave energy by the selected particles occurs. The reference voltage source 20 is set to obtain a null output from the comparison circuit 18. During normal instrument operation, the comparison circuit compares this reference voltage with the receiver output voltage to provide a resultant or difference voltage representing the concentration of the selected particles. As noted earlier, a primary use of the present acoustic instrument is monitoring the concentration of oil in water. The particular instrument 10 shown in FIG. 1 is intended for this application. In this I regard, it is significant to note that oil entrained in water exists in the form of minute droplets having a mean diameter on the order of a few microns. These droplets constitute the particles whose concentration is measured by the instrument.

Referring now in greater detail to the illustrated instrument, the acoustic source 12 comprises an acoustic wave generating transducer 28 within a conduit 30 for conveying the acoustic medium 16, i.e. water, to be examined or monitored and a driver 32 for the transducer. The acoustic receiver 14 comprises an acoustic wave receiving transducer within the conduit opposite the source transducer. The water 16 flows between the transducers, as shown.

The voltage comparison circuit 18 includes a tuned amplifier 33 coupled to the output of the receiving transducer 14, a detector 34 coupled to the output of the amplifier, and a voltage differencer or subtractor 36 coupled to the output of the detector. Amplifier 33 is tuned to a narrow frequency band about the frequency of the driver 32 and amplifies the transducer output over the range of thisfrequency band. The detector 34 converts or rectifies the amplifier output to a corresponding d.c. output voltage which is applied to the differencer 36. The reference voltage source includes a d.c. voltage source 38 coupled to the difference 36 and null adjustment 40 for regulating the dc. reference voltage level to the differencer. The differencer 36 vproduces a resultant dc voltage equal to the difference between the receiver d.c. output voltage and dc. reference voltage applied to the differencer. This resultant or difference voltage is applied to the meter 22, recorder 24, and alarm 26.

In operation of the illustrated oil concentration measuring instrument 10, the driver 32 of the acoustic source 12 is tuned to an acoustic frequency whose half wavelength approximates or is less than the mean diameter of the oil particles or droplets to be measured. This driving frequency may be on the order of 1 megacycle. The instrument is initially calibrated by 0perating the instrument with clean water passing through the conduit 30, that is water of the kind to be acoustically examined but totally free of any oil. During this calibration run, acoustic wave energy at thetuned frequency of the driver 32 is transmitted through the water from ferencer- 36. The'adjustment means 40 of'the reference source is adjusted to obtain a, null output from the differencer.

During normal operation of the instrument 10, the water to be examined or monitored for oil passes through the conduit 30. The acoustic source 12 is again operated to transmit acoustic waveenergy at the tuned frequency of the driver 32 through the waterv to the acoustic receiver 14. During its passage through thewater, the wave energy is attenuated, by scattering-and diffraction, by any oil particles or dropletspresent in the water. It will be understood by those versed inthe art that this scattering and diffraction occurs because of the earlier described relationship between the half.

wavelength of the wave energy and the meanydiameter of the oil droplets. The amount of'attenuation and hence the amplitude of the acoustic wave energy in: cident on and the output ofthe acoustic receiver 14 are proportional to the concentration of the oil dropletsin the water. n I

The dc output voltage from the acoustic receiver 14 and the dc. reference voltage from theireference source 20.are subtractedin the differencer 36 to obtain a difference voltage representing the difference between the output and reference voltages. Since the reference voltage represents the receiver. output with no acoustic attenuation by oil dropletsand the receiver d.c. output voltage represents the receiver output after acoustic attenuation by any oil droplets present in .the water, the difference between these voltages, and hence the difference voltage from the differencer represent the oil concentration in the water. The meter 22 ,is calibrated to .display and the recorder'24 is calibrated to record the oil concentration. The alarm 26 is set to be triggered at. a preset difference voltage level corresponding to a selected or critical oil concentration.

In many oil concentration measurement applications, the water may contain secondary contaminants in addition to and of a larger mean diameter than the oil droplets, such as sand, bits of rock, etc. The modified acoustic measuring instrument 100 of FIG. 2 is designed for use in these latter applications. Instrument 100 has two. acoustic sources 102, 104 and two acoustic receivers 106, 108. The acoustic sources include acoustic transducers 110, 112 arranged side by side within a conduit 114 and drivers 116, 118 for the transducers. The acoustic receivers comprise acoustic transducers arranged side by side within the conduit, directly opposite the source transducers. The receiver outputs are amplified by tuned amplifiers 120, 122, converted to corresponding d.c. output voltages by detectors 124, 126, and then applied to a differencer 128. Amplifiers 120, 122 are tuned to narrow frequency bands about the frequencies of their respective acoustic source 102, 104. The output'of the differencer is applied to a meter 130, recorder 132, and alarm 134. Between the amplifier 120 and detector 124 for the acoustic receiver 106 are null adjustment means 136 for adjusting the receiver output voltageapplied to the detector.

In operation of the instrument 1 00, the water 138 to be acoustically examined passes through the conduit 114 and between the source and receiving transducers 1 10, 112, 106, and 108. Assuming that the water contains only oil droplets and largersecondary contaminants such as sand, bits of rock and the like, the driver 116 of the acoustic source 102 is tuned to an acoustic frequency whose half wavelength approximates or is less than the mean diameter of the oil droplets. The driver 118; of the acoustic. source 104 is tuned to a lower frequency whose half wavelength is substantially larger than the mean diameter of the oil droplets and approximately equal to or less than the mean diameter of the smallest secondary contaminant present in the water. In the case of water from bilges, ballast tanks, and the like, the drivers 116, 118 may be tuned to frequencies on the order of l megacycle and 200 kilocycles, respectively.

The acoustic wave energy from the sources 102, 104 is transmitted through the water to the acoustic receivers 106, 108. From the above discussion of the driving frequencies, it will be understood that the high frequency acoustic waves from the acoustic source 102 are attenuated, by scattering and diffraction, by all the constituents present in the water including the secondary contaminants andany oil droplets entrained in the water. Thelow frequency acoustic wave energy from the acoustic source 104, on the other hand, is attenuated by all the constituents except any oil droplets which may be entrained in the water. .ln other words, the high frequency sees everything present in the water, while the low frequency seesT everything except the oil. Accordingly, the difference in intensity of the wave energy incident on the acoustic receivers represents the attenuation produced by oil dropletsand thus the concentration of the oil droplets,

the dual frequency instrument under discussion,

. thedc. output voltage from the low frequency acoustic receiver 108 which receives acoustic wave energy attenuated by only the secondary contaminants in the water provides a reference voltage. This reference voltage and the d.c. output voltage from the high frequency acoustic receiver 106 are applied to the differeneer 128 which produces a resultant voltage equal to the difference between the output and reference voltages. This difference voltage is applied to the meter 130, recorder 132, and alarm 134.

The dual frequency instrument 100 is initially calibrated by operating the instrument with water passing through the conduit 114 which contains the normal concentration of secondary contaminants but is totally free of oil. Under these conditions, both acoustic frequencies are attenuated only by the secondary contaminants. The null adjustment means 136 is adjusted to a null position wherein the reference voltage from receiver 106 and d.c. output voltage from receiver 108 are equal and the differencer 128 produces a null difference voltage. During normal operation, the high frequency acoustic wave energy is attenuated by any oil present in the water, thereby reducing the d.c. output voltage of the high frequency receiver 106 by an amount related to the oil concentration. The difference voltage from the differencer 128 then represent the oil concentration. The meter 130 and recorder 132 are calibrated to display and record this oil concentration, and the alarm 134 is set to be actuated on response to a preset oil concentration.

It will be immediately evident to those versed in the art that the measuring technique embodied in the dual frequency instrument 1100 may be extended to other frequencies to permit measurement of particle concentrations in several particle diameter ranges. In each measurement, one frequency, the lower reference voltage frequency, is selected to have a half wavelength substantially greater than the mean diameter of the particles to be detected but equal to or less than the diameter of the next larger particles. The higher frequency is selected to have a half wavelength approximating the diameter of the particles to be measured.

What is claimed as new in support of Letters Patent is:

1. An instrument for acoustically examining an acoustic transmission medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, comprising:

an acoustic receiver comprising transducer means to be acoustically coupled to said medium;

an acoustic source comprising transducer means to be acoustically coupled to said medium-for transmitting acoustic wave energy through said medium to said receiver at two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in said medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by an amount related to the concentrations of the constituents, and said first constituent attenuates the acoustic wave energy of the lower frequency by an amount related to the concentration of said first constituent; said receiver producing two output voltages proportional to the incident acoustic wave energy at said two frequencies, respectively; circuit means connected to said receiver for producing a resultant voltage representing the difference between said output voltages; and means for nulling said resultant voltage at a selected concentration of said second constituent in said medium. 2. An acoustic instrument according to claim 1 wherein:

said nulling means comprises means for nulling said resultant voltage with said medium totally devoid of said second constituent, whereby said selected concentration is zero concentration. 3. An acoustic instrument according to claim 1 wherein:

said medium is water, said first constituent is particulate matter such as sand, bits of rock, and the like, and said second constituent is oil. 4. An acoustic instrument according to claim 1 including:

readout means connected to said circuit means for providing a readout corresponding to said resultant voltage. 5. An acoustic instrument according to claim 1 wherein:

said circuit means includes amplifiers tuned to narrow frequency bands about said selected frequencies, respectively, for amplifying the output voltages of said receiver detectors for converting the outputs of said amplifiers to corresponding d.c. output voltages, and differencing means for producing a d.c. voltage representing the 'difference between said d.c. output voltages. 6. An acoustic instrument according to claim 1 wherein:

said acoustic source comprises two separate acoustic transducers which are driven at said selected frequencies, respectively; and said receiver comprises two separated transducers for receiving acoustic wave energy from said source transducers, respectively. 7. An acoustic instrument according to claim 1 wherein:

said nulling means comprises means for nulling said resultant voltage with said medium totally devoid of said second constituent, whereby said selected concentration is zero concentration; said circuit means includes amplifiers tuned to narrow frequency bands about said selected frequencies, respectively, for amplifying the output voltages of said receiver at said driving frequencies, detectors for converting the outputs of said amplifiers to said corresponding d.c. output voltages, and differencing means for producing a d.c. difference voltage representing the difference between said d.c. output voltages;

said source comprises two separated acoustic transducers which are driven at said selected frequencies, respectively; I

said receiver comprises two separated transducers for receiving acoustic wave energy from said source transducers, respectively; and

readout means connected to said differencing means for providing a readout corresponding to said resultant voltage.

8. An acoustic instrument according to claim 7 wherein:

said medium is water, said first constituent is particulate matter such as sand, bits of rock and the like, and said second constituent is oil. 9. The method of acoustically examining a medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, which comprises the steps of:

transmitting through said medium acoustic wave energy of two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in the medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by amounts related to the concentration of the constituents, and said first constituent attenuates the acoustic wave energy of lower frequency by an amount related to the concentration of said first constituent; receiving the acoustic wave energy of each frequency after passage through said medium and producing output voltages related to the received wave energy of the two frequencies, respectively;

producing a resultant voltage representing the difference between said output voltages; and

nulling said resultant voltage at a given concentration of said second constituent, whereby said resultant voltage represents. the concentration of said second constituent in said medium.

10. The method of claim 9 wherein:

said resultant voltage is nulled with said medium totally devoid of said second constituent, whereby said selected concentration is zero concentration.

11. The method of claim 9 wherein: i

said medium is water, said first constituent is particulate matter such as sand, bits of rock and the like, and said second constituent is oil.

12. An instrument for acoustically examining an acoustic transmission medium containing a first particulate constituent having a different acoustic impedancethan the medium .to detect a second particube acoustically coupled tosaid medium; an acoustic source comprising transducer means to v be acoustically coupled to said'medium for transmitting acoustic wave energy through said medium to said receiver at two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in said medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by an amount related to the concentrations of the constituents, and said first constituent attenuates the acoustic'wave energy of the lower frequency by an amount related to the concentration of said first constituent; and

said receiver producing two output voltages proportional to the incident acoustic wave energy at said two frequencies, respectively, the difference between said output voltages being proportional to the concentration of said second constituent. 13. The method of acoustically examining a medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, which comprises the steps of:

transmitting through said medium acoustic wave energy of two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said lattervparticle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in the medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by amounts related to the concentration of the constituents, and said first constituent attenuates the acoustic wave energy of lower frequency by an amount related to the concentration of said first constituent; and receiving the acoustic wave energy of each frequency after passage through said medium and producing output voltages related to the received wave energy of the two frequencies, respectively, the difference between said output voltages being proportionalto the concentration of said second constituent. 

1. An instrument for acoustically examining an acoustic transmission medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, comprising: an acoustic receiver comprising transducer means to be acoustically coupled to said medium; an acoustic source comprising transducer means to be acoustically coupled to said medium for transmitting acoustic wave energy through said medium to said receiver at two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in said medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by an amount related to the concentrations of the constituents, and said first constituent attenuates the acoustic wave energy of the lower frequency by an amount related to the concentration of said first constituent; said receiver producing two output voltages proportional to the incident acoustic wave energy at said two frequencies, respectively; circuit means connected to said receiver for producing a resultant voltage representing the difference between said output voltages; and means for nulling said resultant voltage at a selected concentration of said second constituent in said medium.
 2. An acoustic instrument according to claim 1 wherein: said nulling means comprises means for nulling said resultant voltage with said medium totally devoid of said second constituent, whereby said selected concentration is zero concentration.
 3. An acoustic instrument according to claim 1 wherein: said medium is water, said first constituent is particulate matter such as sand, bits of rock, and the like, and said second constituent is oil.
 4. An acoustic instrument according to claim 1 including: readout means connected to said circuit means for providing a readout corresponding to said resultant voltage.
 5. An acoustic instrument according to claim 1 wherein: said circuit means includes amplifiers tuned to narrow frequency bands about said selected frequencies, respectively, for amplifying the output voltages of said receiver detectors for converting the outputs of said amplifiers to corresponding d.c. output voltages, and differencing means for producing a d.c. voltage representing the difference between said d.c. output voltages.
 6. An acoustic instrument according to claim 1 wherein: said acoustic source comprises two separate acoustic transducers which are driven at said selected frequencies, respectively; and said receiver comprises two separated transducers for receiving acoustic wave energy from said source transducers, respectively.
 7. An acoustic instrument according to claim 1 wherein: said nulling means comprises means for nulling said resultant voltage with said medium totally devoid of said second constituent, whereby said selected concentration is zero concentration; said circuit means includes amplifiers tuned to narrow frequency bands about said selected frequencies, respectively, for amplifying the output voltages of said receiver at said driving frequencies, detectors for converting the outputs of said amplifiers to said corresponding d.c. Output voltages, and differencing means for producing a d.c. difference voltage representing the difference between said d.c. output voltages; said source comprises two separated acoustic transducers which are driven at said selected frequencies, respectively; said receiver comprises two separated transducers for receiving acoustic wave energy from said source transducers, respectively; and readout means connected to said differencing means for providing a readout corresponding to said resultant voltage.
 8. An acoustic instrument according to claim 7 wherein: said medium is water, said first constituent is particulate matter such as sand, bits of rock and the like, and said second constituent is oil.
 9. The method of acoustically examining a medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, which comprises the steps of: transmitting through said medium acoustic wave energy of two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in the medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by amounts related to the concentration of the constituents, and said first constituent attenuates the acoustic wave energy of lower frequency by an amount related to the concentration of said first constituent; receiving the acoustic wave energy of each frequency after passage through said medium and producing output voltages related to the received wave energy of the two frequencies, respectively; producing a resultant voltage representing the difference between said output voltages; and nulling said resultant voltage at a given concentration of said second constituent, whereby said resultant voltage represents the concentration of said second constituent in said medium.
 10. The method of claim 9 wherein: said resultant voltage is nulled with said medium totally devoid of said second constituent, whereby said selected concentration is zero concentration.
 11. The method of claim 9 wherein: said medium is water, said first constituent is particulate matter such as sand, bits of rock and the like, and said second constituent is oil.
 12. An instrument for acoustically examining an acoustic transmission medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, comprising: An acoustic receiver comprising transducer means to be acoustically coupled to said medium; an acoustic source comprising transducer means to be acoustically coupled to said medium for transmitting acoustic wave energy through said medium to said receiver at two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in said medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by an amount related to the concentrations of the constituents, and said first constituent attenuates the acoustic wave energy of the lower frequency by An amount related to the concentration of said first constituent; and said receiver producing two output voltages proportional to the incident acoustic wave energy at said two frequencies, respectively, the difference between said output voltages being proportional to the concentration of said second constituent.
 13. The method of acoustically examining a medium containing a first particulate constituent having a different acoustic impedance than the medium to detect a second particulate constituent having an acoustic impedance differing from those of both said medium and first constituent and a mean particle diameter smaller than the first constituent, which comprises the steps of: transmitting through said medium acoustic wave energy of two different selected acoustic frequencies such that the higher frequency has a half wavelength approximating the mean particle diameter of said second constituent and the lower frequency has a half wavelength substantially greater than said latter particle diameter and approximating or less than the mean particle diameter of said first constituent, whereby both constituents when present in the medium attenuate the acoustic wave energy of higher frequency by scattering and diffraction by amounts related to the concentration of the constituents, and said first constituent attenuates the acoustic wave energy of lower frequency by an amount related to the concentration of said first constituent; and receiving the acoustic wave energy of each frequency after passage through said medium and producing output voltages related to the received wave energy of the two frequencies, respectively, the difference between said output voltages being proportional to the concentration of said second constituent. 